Light interference device with low degree of spacial coherence

A light interference device for measuring the flatness of a surface to be measured, said surface to be measured being one side of a plate of transparent material having a back face, said device comprising a light source, an optical element having a reference surface for interference upon which a parallel light bundle from the light source impinges and passes through and from which a parallel light bundle is reflected, said reference surface being locatable adjacent but spaced from said surface to be measured whereby interference fringes are generated as a result of the interaction of light rays reflected from said reference surface and from said surface to be measured, said light source having a low degree of spatial coherence whereby said interference fringes are localized near said interference surface, an optical system between said light source and said reference surface for providing a parallel light bundle coming out from said reference surface toward said surface to be measured at an angle .theta. within the range of 0.degree. <

• .theta. .ltoreq. 30.degree., an observing surface, and an optical projection system for projecting interference fringes localized near said interference surface onto said observing surface.